1. Field of the Invention
This invention relates to a brake system for a vehicle, which attains such objects as versatile and sophisticated control and improved pedal feel with a minimum number of parts.
2. State of the Prior Art
One typical example of prior art brake system capable of both antilock control and brake traction control is shown in FIG. 4 of U.S. Pat. 3,667,813.
Although this prior art publication mainly describes a control method, since the basic elements of this prior art are concisely shown in its FIG. 4, the present invention is conveniently compared with this figure. Therefore, this figure is reproduced as FIG. 2 of the present invention to describe the prior art.
In this prior art, the output of a brake fluid pressure generator 27, adjusted by manual control, is shut off by a solenoid valve 31. Instead, a dynamic pressure from a dynamic pressure source having a motor 24, a pump 25 and an accumulater 26 is introduced into a brake unit 20 through a solenoid valve 19, or pressure fluid therefrom is discharged into a reservoir 23 through a solenoid valve 30, to adjust the braking pressure.
Namely, three solenoid valves 31, 19 and 30, provided for each brake circuit, are controlled by commands from an electronic control unit to obtain a desired brake pressure irrespective of the manual control.
One problem of the prior art brake system shown in FIG. 2 is that once the solenoid valve 31 is shut off, the brake unit 20 is completely separated from the manual control, so that the system cannot quickly respond to changes of the intention of the driver. Namely, in this system, once the solenoid valve 31 is closed, the system is controlled to attain the maximum deceleration or acceleration irrespective of the intention of the driver until the change of the intention of the driver is transmitted to a brake switch 33, i.e. a pedal 28 is moved forward or backward to such an extent as to actuate the switch 33.
Of course, it is desirable to quickly reflect the change of the intention of the driver during antilock control or automatic brake control, such as traction control. The prior art brake system shown in FIG. 2 cannot meet this requirement.
To improve the pedal feel, it is necessary both to shorter the pedal stroke in normal conditions and to lighten the pedal force in case of failure of the dynamic pressure source. But the system arrangement shown in FIG. 2 cannot meet this requirement either.